The present invention relates generally to power generation and distribution systems and methods. More particularly, the present invention relates to systems and methods for providing power through parallel operation of multiple power sources.
The United States Electric Power Research Institute (EPRI), which is the uniform research facility for domestic electric utilities, predicts that up to forty percent of all new electric power generation could be provided by distributed generators in the next few years. In other words, the traditional power grid that is used to provide power to consumers of electricity will increasingly be augmented or replaced with generators that may be located at the site of the demand or at a remote site and coupled via distribution lines.
Consumers of electricity can benefit from having a power generating system which uses multiple power sources. For instance, using multiple power sources provides a measure of redundancy in case one or more of the power sources becomes temporarily unavailable (i.e. “brownouts” and “blackouts” which are prevalent in many parts of the world). Besides providing redundancy, consumers may recognize a cost savings when using generators to augment or replace the power grid. Using multiple smaller generators allow the optimization of efficiency and/or system reliability based on the power demand at the site. Accordingly, power generating systems are increasingly using multiple power sources to provide power for consumers' loads.
When providing power using multiple power sources, the sources are often connected in parallel to the load. These power sources may include the traditional power utility grid. Depending on utility billing methods, it is quite possible to have significant reductions in utility costs i.e. peak shaving or improving power factor of energy drawn from the utility grid. However, some consumers choose to forego the traditional power grid entirely and instead solely use a number of power generators which could be operated in parallel, to meet their power demands. For instance, ten generators, each capable of providing a maximum of 75 kilowatts of power, could be operated in parallel to provide 750 kilowatts of power to a load.
Loads are often designed to operate using power having particular parameters such as a specific frequency, phase, and voltage. These loads also may have an acceptable range of deviation about the designed for value; power provided outside this acceptable range may degrade the performance of the load or even cause damage. Accordingly, it is desirable to provide power that has the proper values for these parameters or that is within the acceptable range based on the load's tolerance for deviations. Additionally, when power sources are operated in parallel, if the sources provide power having different parameters (i.e. different frequencies or voltages), then the sources will also act as loads to each other and the sources may receive unwanted power feedback from the other power sources. If power sources provide power having different frequencies and voltages, even for brief intervals, extreme transients in current and energy flows may be caused. This unwanted feedback may cause damage to the power sources and/or loads or degrade their performance.
Traditionally, if a group of generators is used to power a load, then one of the generators (often termed the master) provides the frequency, phase, and/or voltage for the other generators to use so that these parameters may be synchronized. For instance, one approach of operating multiple microturbine generators in parallel is to phase-lock each of the generators to a master oscillator. A synchronization signal (e.g., a synchronization frequency such as a 60 Hz signal) may be sent to each of the generators, such that each generator will phaselock onto the synchronization signal, ensuring that the voltage produced by each generator is exactly in-phase with the other generators. Similarly, if a load is powered by a combination of the grid and one or more generators, the generators would obtain the frequency, phase, and voltage to use for providing power to the load from sensing the parameters of the power delivered by the grid.
These power sources are often monitored for compliance with the acceptable range for power parameters and disconnected from the load when out of compliance. For instance, if the grid provides power that becomes out of the acceptable range for the parameters for the load, then it may be disconnected from the load. Likewise, if the generator that is providing the parameters for the other generators to use for power production becomes non-compliant and must be disconnected, then all of the associated generators also are disconnected and become unavailable to the load. Generators, including those acting as masters which may provide the synchronization parameters such as frequency, have many moving and mechanical parts thus increasing the likelihood of their breakdown and mechanical failure. This may cause the loss of power to the load since no valid reference signal is available to the remaining generators. Further, the power generating system may be unable to meet the power demands of the consumer with only the remaining power sources.
Even if a disconnected power source returns to compliancy (i.e. having power with parameters within the acceptable range) with the parameter ranges, the other power sources often must be shut down before the disconnected power source can be reconnected. This shutdown is often necessary because the power provided by the remaining connected power sources may be at a value different than the disconnected power source that is now again within the acceptable range. If the source is reconnected without synchronizing the values such as frequency, phase, and/or voltage, one or more of the power sources may receive unwanted power feedback. This shutdown may cause the user to suffer losses from the shutdown in lost time and start-up costs.
Further, when sources are connected in parallel and one source is providing sufficient power to the load, the system may be unaware if other power sources become unavailable. Thus, the power system could be unaware that it is unable to meet increased demand that outstrips the ability of the remaining power sources. Further, the system could be unaware if a graceful shutdown is needed in the event of the failure of the remaining power sources. The need for shutdowns may be decided by the remaining generators becoming overloaded for a defined time interval. Also, large overloads may cause high currents and low voltages which also may require a shutdown.
Therefore, a need exists for systems and associated methods that allow for individual power sources to be disconnected and reconnected without interrupting the power supply from the other power sources and that allow for these power sources to provide power at substantially the same power parameters such as frequency, phase, and/or voltage. A further need exists for systems and methods that allow for the power system easily to determine if a power source becomes unavailable.